1. Field of the Invention
The present invention relates to transflective display devices which display an image in both of a transmissive region and a reflective region.
2. Description of the Related Art
Liquid crystal display devices have been widely used in electronic devices such as a monitor, a projector, a cellular phone, a personal digital assistance (hereinafter, referred to as PDA), recently.
Such liquid crystal display devices are classified into reflective, transmissive, and transflective types. The reflective liquid crystal display devices guide surrounding light to the inside of a liquid crystal panel, and reflect this light by a reflective member, thereby obtaining display light.
The transmissive liquid crystal display devices are configured to emit light from a light source (hereinafter, referred to backlight) disposed on the back of a liquid crystal panel through the liquid crystal panel.
According to the transflective liquid crystal display devices, transmissive display using light from the backlight is mainly observed under relatively dark environments such as an indoor environment. Under relatively bright environments such as an outdoor environment, a reflective display using surrounding light is mainly observed. As a result, a display with a high contrast ratio can be provided regardless of surrounding brightness.
That is, the transflective liquid crystal display devices can provide display under all environments regardless of indoor and outdoor environments, and therefore such devices have been often equipped with mobile devices such as a cellular phone, a PDA, and a digital camera.
According to such transflective liquid crystal display devices, the liquid crystal panel has two different display regions: a reflective region; and a transmissive region.
In the transmissive region, light from a backlight is passed through a liquid crystal layer and a color filter only one time and then emitted.
In the reflective region, surrounding light transmitted through the color filter and the liquid crystal layer is reflected by a reflective member and passed through the liquid crystal layer and the color filter again, and then emitted.
Thus, the transmissive region and the reflective region use different light sources, that is, backlight and surrounding light, thereby generating display light.
A color filter used in a conventional liquid crystal display device includes three primary colors of RGB (red, green, and blue) (hereinafter, referred to as three-color filter). However, by using only these three primary colors, a color reproduction range is not sufficiently extended. Therefore, not all colors which can be recognized by human eyes can be expressed.
Four or more-color filters have been proposed in order to extend the color reproduction range and improve use efficiency of light.
For example, Patent Document 1 (Japanese Kokai Publication 2001-209047 (day of publication: Aug. 3, 2001)) discloses a color filter including four colors of RGBY in which Y (yellow) is added to the three primary colors of RGB (hereinafter, also referred to as four-color filter). With respect to this RGBY, R and G, and B and Y, are mutually opposite, respectively, and such combinations are complementary color combinations based on human visual characteristics.
[Non-patent Document 1] MacCamy, C. S., Correlated Color Temperature As An Explicit Function of Chromaticity Coordinates, Color Res. Appl. 17, 142-144 (1992).
White balance is mentioned as one important display performance characteristic of display devices. This white balance is a hue of white displayed by a display device, and mainly determined by a color tone of a light source and a configuration of a color filter.
An LED or a CCFT (cold cathode fluorescent tube) having an emission peak to a spectrum transmittance of a three-color filter is generally used as a backlight of liquid crystal display devices.
However, use of the above-mentioned four-color filter in a transflective liquid crystal display device largely affects the white balance, although a luminance or a color reproduction range can be improved.
That is, if the four-color filter is used as a color filter in a liquid crystal display device including a backlight corresponding to the three-color filter, the color tone of the white balance is shifted to yellow.
The white balance in the transmissive display can be improved by adjusting the color tone of the backlight.
In the reflective display using surrounding light, however, it can be impossible to adjust the color tone of the light source and therefore it is difficult to suppress the white balance from shifting to yellow.
The adjustment of the color tone of the backlight causes a large difference in white balance between the transmissive display and the reflective display.
It can be possible that the white balance of the display device is previously adjusted by four colors of RGBY in such a way that optimum white balance can be obtained when surrounding light is used.
However, a B filter needs to have an extremely thin thickness for such adjustment. Even if the B filter has a thin thickness, white balance which is absolutely not shifted to yellow can not be obtained.
This adjustment also causes a large difference between a display color and a color obtained using the three-color filter in accordance with NTSC.